It is proposed to develop and validate serial electrocardiographic change criteria for interim myocardial infarction in population studies and clinical trials. The possibility for bias-free and quantitative assessment of differences between coparison groups is central to the design, operation and interpretation of such studies. TECG provides a graphic record of cardiovascular findings and events and is particularly amenable to central measurement and handling by properly blinded, standardized and quantitative procedures. Despite the recognized value of the ECG for epidemiological studies and trials, no formalized body of quantitative criteria for significant serial ECG change has been produced and validated. Clinical ECG criteria for serial change remain impressionistic. Prior clinical-pathologic validation has defined new significant change suggestive of intervening non-fatal myocardial infarction as a two class or greater change in Minnesota Q/QS code (i.e. from 1.0 to 1.2, or 1.3 to 1.1). However, application in incident cases will be identified by systematic simultaneous comparison of serial ECGs to confirm ECG "worsening." Criteria will be validated by evidence independent of the ECG. A valid serial ECG classification system would enhance diagnostic specificity for CHD incident events as endpoints in clinical trials and population studies. This would make treatment group comparisons more precise, reduce bias and improve sample size estimates. Additionally, this research will study the prognostic significance of serial ECG change.